Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems, operating at wavelengths around 248 nm, as well as ArF-excimer laser systems, which operate at around 193 nm. Vacuum UV (VUV) tools are based on F2-laser systems operating at around 157 nm. These relatively short wavelengths are advantageous for photolithography applications because the critical dimension, which represents the smallest resolvable feature size that can be produced photolithographically, is proportional to the wavelength used to produce that feature. The use of smaller wavelengths can provide for the manufacture of smaller and faster microprocessors, as well as larger capacity DRAMs, in a smaller package. In addition to having smaller wavelengths, such lasers have a relatively high photon energy (i.e., 7.9 eV) which is readily absorbed by high band gap materials such as quartz, synthetic quartz (SiO2), Teflon (PTFE), and silicone, among others. This absorption leads to excimer and molecular fluorine lasers having even greater potential in a wide variety of materials processing applications. Excimer and molecular fluorine lasers having higher energy, stability, and efficiency are being developed as lithographic exposure tools for producing very small structures as chip manufacturing proceeds into the 0.18 micron regime and beyond. Master Oscillator Power Amplifier (MOPA) excimer laser systems have an advantage of power scalability combined with improved spectral parameters, since power scaling is not traded off for spectral narrowness, as is the case in a traditional single-oscillator laser. Thus, the MOPA concept is becoming a mainstream route to increased throughput of chip manufacture, with ever increasing degree of minituarization.
The desire for such submicron features comes with a price, however, as there is a need for improved processing equipment capable of consistently and reliably generating such features. Further, as excimer laser systems are the next generation to be used for micro-lithography applications, the demand of semiconductor manufacturers for powers of 40 W or more to support throughput requirements leads to further complexity and expense.
Excimer laser systems have the potential to meet the target performance on spectral purity and high average power as required for applications such as microlithography. Such laser systems must deliver very high spectral purity, as well as a high average power of at least 40 W in order to support the throughput requirements of advanced lithography scanner systems. In many microlithography and other applications the laser is triggered by the scanner in order to correlate the light pulse with the condition of the scanner. A typical trigger pattern is a burst with a varying on/off ratio. Several of the laser pulse parameters, such as the pulse energy and pulse spectrum, vary in the beginning of the burst, and are influenced by the change in the burst pattern. At the same time, these laser pulse parameters can be of critical importance to the process. The time necessary for the laser system to adapt to a change in the requested laser parameter or trigger pattern can lead to a reduction of the system availability, such that it can be desirable to eliminate that time delay.
Further, current optical microlithography processes allow printing of extremely small feature sizes for integrated circuits, with a 65 nm node being in development at this time. At this level of resolution level, however, the influence of other laser parameters, such as the pointing instability of the output laser beam, becomes non-negligible. Output beam pointing can change by as much as several hundred micro-radians within a burst of several hundred pulses. Existing systems are not able to correct for such an error on a time scale that is less than time interval between pulses, such as 160 microseconds at repetition rate of 6 kHz.